In order that a base metal, such as Ni or a Ni alloy, can be used as a conductive material constituting internal electrodes for a monolithic ceramic capacitor, the dielectric ceramic composition constituting dielectric ceramic layers is required not to be converted into a semiconductor even when fired at a low oxygen partial pressure. Furthermore, the dielectric ceramic composition is required to have the characteristic of a flat relative dielectric constant with respect to temperature. Various dielectric ceramic compositions that can satisfy the requirements have been reported.
A dielectric ceramic composition of interest to the present invention is described in, for example, Japanese Unexamined Patent Application Publication No. 2005-194138 (Patent Document 1). Patent Document 1 discloses a dielectric ceramic composition represented by a composition formula: 100(Ba1-xCax)mTiO3+aMnO+bV2O5+cSiO2+dRe2O3 (wherein Re represents at least one metal element selected from Y, La, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb; and a, b, c, and d each represent moles), the dielectric ceramic composition satisfying the requirements:    0.030≦x≦0.20,    0.990≦m≦1.030,    0.010≦a≦5.0,    0.050≦b≦2.5,    0.20≦c≦8.0, and    0.050≦d≦2.5.
Monolithic ceramic capacitors are used in, for example, car-mounted products in addition to other common consumer products. The properties required for monolithic ceramic capacitors for use in common consumer products are not necessarily the same as those in car-mounted products.
For example, common consumer products are required to satisfy X7R characteristics specified by the EIA standard (the rate of change of the dielectric constant with temperature is within the range of ±15% in the temperature range of −55° C. to 125° C. with reference to 25° C.) with respect to electrostatic capacity temperature characteristics. Car-mounted products are required to satisfy X8R characteristics specified by the EIA (the rate of change of the dielectric constant with temperature is within the range of ±15% in the temperature range of −55° C. to 150° C. with reference to 25° C.).
With respect to reliability as measured in a high-temperature loading test, common consumer products are evaluated at 150° C. Car-mounted products are evaluated at 175° C. Note that the load voltage used is determined depending on the intended applications of products including monolithic ceramic capacitors in such testing.
The dielectric ceramic composition described in Patent Document 1 is not necessarily suited to constitute dielectric ceramic layers in car-mounted monolithic ceramic capacitors.
That is, the dielectric ceramic composition described in Patent Document 1 has high reliability, i.e., a mean time to failure of 100 hours or more, in the high-temperature loading test in which a direct-current voltage is applied at 150° C. so as to produce a field strength of 10 V/μm. However, Patent Document 1 does not consider the reliability of the dielectric ceramic composition in a high-temperature loading test in which a direct-current voltage is applied at 175° C. so as to produce a field strength of 50 V/μm.
In the case where the Re content is increased the dielectric ceramic composition described in Patent Document 1, in order to improve reliability, the rate of change of the relative dielectric constant with temperature is degraded. It is thus impossible to achieve a balance between high reliability and a low rate of change of the relative dielectric constant with temperature.
Furthermore, it was found that although the rate of change of the relative dielectric constant with temperature satisfied an absolute value of 15% or less in the range of −55° C. to 125° C. for the dielectric ceramic composition described in Patent Document 1, the rate of change of the relative dielectric constant with temperature did not satisfy an absolute value of 15% or less in the range of −55° C. to 150° C.
Patent Document 1 is Japanese Unexamined Patent Application Publication No. 2005-194138